In general, a gear cutting process includes: forming a gear by cutting a predetermined gear material; heat-treating the machined gear; and then finishing (tooth profile grinding) the gear to remove distortion and the like due to this heat treatment. Heretofore, various tooth profile grinding methods using tools such as WA grinding wheels and superabrasive (diamond, CBN, and the like) grinding wheels have been provided to efficiently finish the tooth surfaces of a gear after heat treatment. Moreover, the forms of tools used in these methods are external gear forms, internal gear forms, screw (worm) forms, and the like which are chosen in accordance with the form of a gear to be ground.
On the other hand, among gears, internal gears are frequently used in automotive transmissions and the like. Recently, there has been a demand for the improvement in the machining accuracy of the gears for the purpose of reducing the vibration and noise of the transmissions.
In this regard, as one of the methods of grinding an internal gear after heat treatment, an internal gear machining method is provided in which grinding of tooth surfaces of an internal gear is performed using a barrel-shaped threaded grinding wheel by synchronously rotating the internal gear and the barrel-shaped threaded grinding wheel in mesh with each other with a crossed-axes angle formed therebetween. In this internal gear machining method, the meshing rotation and the crossed-axes angle of the internal gear and the barrel-shaped threaded grinding wheel generate a slip velocity (grinding velocity) between the internal gear and the barrel-shaped threaded grinding wheel, and this allows the tooth surfaces of the internal gear to be ground by the barrel-shaped threaded grinding wheel.